Mercury is a liquid and allows the flow of electricity, though it is not an electrolyte.

Options

A. Breaks up into cations and anions
B. Due to the presence of free electrons in its penultimate shell
C. New substance is formed
D. All of the above

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question states that mercury is a liquid that allows the flow of electricity but is not classified as an electrolyte. We need to explain why this is the case.

2. **Defining Electrolytes**: An electrolyte is a substance that dissociates into ions when dissolved in a solvent, allowing it to conduct electricity. Common examples include salts like sodium chloride (NaCl), which dissociate into Na⁺ and Cl⁻ ions in solution.

3. **Properties of Mercury**: Mercury is a metal that is liquid at room temperature. Unlike typical electrolytes, mercury does not dissociate into ions. Instead, it contains free electrons that allow it to conduct electricity.

4. **Conductivity Mechanism**: The conductivity in mercury arises from the presence of free electrons in its atomic structure. These electrons can move freely, allowing electric current to pass through the liquid.

5. **Comparison with Electrolytes**: In electrolytes, electric current is carried by the movement of ions (cations and anions). In contrast, in mercury, the current is carried by the movement of free electrons, not ions. Therefore, mercury does not meet the criteria to be classified as an electrolyte.

6. **Conclusion**: Mercury can conduct electricity due to its free electrons, but it does not dissociate into ions, which is why it is not considered an electrolyte.

### Final Answer:
Mercury is a liquid that conducts electricity due to the presence of free electrons in its atomic structure. It is not classified as an electrolyte because it does not dissociate into ions like typical electrolytes do.

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